First Degree Heart Block

Definition

• There is delay, without interruption, in conduction from atria to ventricles
• ‘Marked’ first degree heart block is present if PR interval > 300ms

Causes of First Degree Heart Block

• Increased vagal tone
• Athletic training
• Inferior MI
• Mitral valve surgery
• Myocarditis (e.g. Lyme disease)
• Electrolyte disturbances (e.g. Hyperkalaemia)
• AV nodal blocking drugs (beta-blockers, calcium channel blockers, digoxin, amiodarone)
• May be a normal variant

Clinical significance

• As an isolated finding this is a benign entity that does not cause haemodynamic instability
• No specific treatment is required

ECG Examples

Example 1

• Marked first degree heart block
• PR interval > 300 ms, P waves are buried in the preceding T wave

Example 2

• Sinus bradycardia with 1st degree AV block
• PR interval > 300 ms

Example 3

• Normal sinus rhythm with 1st degree AV block
• PR interval 260 ms

Related Topics

• AV block: 2nd degree, Mobitz I
• AV block: 2nd degree, Mobitz II
• AV block: 2nd degree, “fixed ratio blocks” (2:1, 3:1)
• AV block: 2nd degree, “high grade AV block”
• AV block: 3rd degree (complete heart block)

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
• Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
• Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Zimmerman FH. ECG Core Curriculum. 2023
• Mattu A, Berberian J, Brady WJ. Emergency ECGs: Case-Based Review and Interpretations, 2022
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Hampton J, Adlam D. The ECG Made Practical 7e, 2019
• Kühn P, Lang C, Wiesbauer F. ECG Mastery: The Simplest Way to Learn the ECG. 2015
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest